1. Field of the Invention
The present invention relates to a polyphase direct-current motor, and more particularly to a three-phase direct-current core-type motor which minimizes cogging and hence reduces vibration and noise during operation.
2. Prior Art
Recently, polyphase direct-current motors such as three-phase direct-current motors having armature cores are widely used in various applications such as drum motors and capstan motors in video tape recorders (VTR), for example.
As the slots in the core of a polyphase direct-current motor move past the boundaries between field poles (i.e., N and S poles), the motor suffers torque and speed variations known as cogging. The mechanism which generates cogging will be described below with respect to a three-phase direct-current motor.
Generally, the number P of field poles and the number N of slots (protruding poles) in a three-phase direct-current motor are given by: EQU P=(3.+-.1)n, N=3n
where n is an integer of 1 or more. Tables 1 and 2 below show the frequencies of cogging per revolution of the rotor of a three-phase direct-current motor. Tables 1 and 2 indicate that the cogging frequency F is represented by the least common multiple of P and N, and the number of positions K where cogging takes place at the same time is represented by the greatest common measure of P and N.
TABLE 1 ______________________________________ [P = (3 + 1)n] n P N F K ______________________________________ 1 4 3 12 1 2 8 6 24 2 3 12 9 36 3 4 16 12 48 4 5 20 15 60 5 6 24 18 72 6 7 28 21 84 7 8 32 24 96 8 . . . . . . . . . . . . . . . ______________________________________
TABLE 2 ______________________________________ [P = (3 - 1)n] n P N F K ______________________________________ 1 2 3 6 1 2 4 6 12 2 3 6 9 18 3 4 8 12 24 4 5 10 15 30 5 6 12 18 36 6 7 14 21 42 7 8 16 24 48 8 . . . . . . . . . . . . . . . ______________________________________
The manner in which cogging is generated in a three-phase direct-current motor will be described with reference to FIGS. 15 through 17 of the accompanying drawings, where n=4 (i.e. 8 field poles and 12 armature slots).
As shown in FIG. 15, a three-phase direct-current motor 100 comprises an armature having a core 100C with coils 100L wound in 12 slots between protruding or salient poles c.sub.1 through c.sub.12, and a cylindrical field magnet having 8 radially magnetized field magnetic poles 100M, the field magnet being radially spaced with an air gap G from the core 100C. A cylindrical yoke Y of iron for providing a magnetic path is fitted over the cylindrical field magnet. In operation, either the field magnet or the armature is fixed, whereas the other is rotated.
When the motor is energized, cogging is produced as the slots move successively past the boundaries a through h between the N and S poles 100M of the field magnet, resulting in torque or speed variations as shown in FIG. 17 at (a) through (h) at the respective magnetic pole boundaries (a) through (h). The total cogging of the motor 100 has a waveform as shown in FIG. 17 at (i), and occurs at 24 cycles per revolution, which cycles are equivalent to the least common multiple of P (=8: the number of field poles) and N (=12: the number of slots).
As described above, the number of positions K where cogging takes place at the same time is indicated by the greatest common measure of P and N, and hence is 4. The number K can also be expressed by: EQU K=N.times.P/the least common multiple of N and P (1)
If it is assumed that the magnitude of cogging which takes place at each of the pole boundaries a through h is represented by t, then the magnitude T of the entire cogging of the motor 100 at any one time is indicated by: EQU T=Kt=12=8t/24=4t
Therefore, the magnitude of the total cogging of the motor 100 is four times the magnitude of the cogging at each of the pole boundaries a through h.
As shown in FIG. 16, the protruding poles c.sub.1, c.sub.4, c.sub.7, c.sub.10 are in phase with the field magnetic poles 100M. Therefore, if the coil 100L is wound clockwise (CW) around the protruding pole c.sub.1, then it is also wound CW around each of the protruding poles c.sub.4, c.sub.7, c.sub.10, these coils constituting U-phase coils. Similarly, V-and W-phase coils are provided by the coils wound around the protruding poles c.sub.2, c.sub.5, c.sub.8, c.sub.11 and the protruding poles c.sub.3, c.sub.6, c.sub.9, c.sub.12, respectively. In FIG. 16, the suffix s of each phase indicates the winding start, whereas the suffix e indicates the winding end.
The phase difference .phi. between two adjacent protruding poles with respect to the field magnetic poles 100M is expressed by an electrical angle which is given as follows: EQU .phi.=(360.degree./N).times.(P/2) (2)
For example, the phase difference between two adjacent protruding poles with the U- and V-phase coils wound respectively therearound, e.g., the protruding poles c.sub.1 and c.sub.2, is an electrical angle of 120.degree. because .phi.=(160.degree./12).times.8/2=120.degree.. Likewise, the phase difference between two adjacent protruding poles with the V- and W-phase coils wound respectively therearound, and the phase difference between two adjacent protruding poles with the W-and U-phase coils wound respectively therearound are each an electrical angle of 120.degree..
When currents of three phases, i.e., U, V, and W phases, which are out of phase with each other by an electrical angle of 120.degree. are supplied to the U-, V-, and W-phase coils, a continuous torque is developed between the armature and the field magnet, thus rotating the three-phase direct-current motor 100. Since the four protruding poles in each set are held in phase with the magnetic poles 100M, the magnitude of the total cogging of the motor 100 at any one time is about four times the magnitude of the cogging produced at each slot, as described above.
Therefore, the magnitude of the total cogging of the motor 100 at any one time is equal to the product of the magnitude of the cogging produced at each slot and the number of places where cogging is developed at the same time. Since the number of such places in the conventional motor is large, the produced total cogging is also large.
As described above in Tables 1 and 2, as the number n increases, the number of positions where simultaneous cogging takes place also increases. Therefore, if the number of poles P and the number of slots (protruding poles) N are increased in order to reduce torque ripples, then the magnitude of cogging is also increased.
In cases where the motor 100 is employed as a drum motor or a capstan motor in a VTR, jitter or wow and flutter are increased in signals produced by the VTR, or noise and vibration are generated.
Cogging may be reduced by varying the waveform of a current used to magnetize the field magnet or defining auxiliary slots in or poles on the core 100C. However, it is difficult to establish conditions for varying the waveform of a magnetizing current. If auxiliary slots are defined in the core, then the torque generated by the motor will be reduced since the total amount of magnetic fluxes is reduced Auxiliary poles are also disadvantageous, particularly with respect to smaller motors, in that the auxiliary poles reduce the space available for the winding of coils, resulting in a reduction in the torque generated.